I have been looking into the httpd.service unit file, and found this:
ExecStop=/bin/kill -WINCH ${MAINPID}
# We want systemd to give httpd some time to finish gracefully, but still want
# it to kill httpd after TimeoutStopSec if something went wrong during the
# graceful stop. Normally, Systemd sends SIGTERM signal right after the
# ExecStop, which would kill httpd. We are sending useless SIGCONT here to give
# httpd time to finish.
KillSignal=SIGCONT
I am pretty sure that (at least in more recent systemd versions) you want to use:
KillSignal=SIGWINCH
KillMode=mixed
This will send SIGWINCH to the main pid when the unit is supposed to go down, and then after a timeout send SIGKILL to everything that is remaining in the entire cgroup.
(Note that KillMode=mixed is the key here, since it does the first kill only on the main process, and the second kill on the entire cgroup. It is a relatively new addition though, so please be careful before backporting such a unit file change too eagerly...)

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